A thermal printer is a device capable of printing characters, bar codes or other marks on a thermal print medium. Printing is accomplished by raising the temperature of the thermal print medium above a threshold or conversion temperature, whereupon a coating on the thermal print medium undergoes a chemical change and changes color. Typically, the temperature of the thermal print medium is raised by the use of a thermal print head that includes one or more resistive print elements that are mounted on a ceramic substrate and that are maintained in contact with the thermal print medium. The configuration of each print element defines a portion of a character, or an entire character, to be printed.
It is important that a thermal printer be capable of precisely controlling the amount of heat applied to print each character portion. Control of the amount of heat applied to the thermal print medium is achieved, in part, by controlling the exposure time, i.e., the time during which the thermal print medium is held above the conversion temperature. An effective technique for controlling exposure time is described in U.S. Pat. No. 4,391,535. In the technique described therein, a driver circuit provides energy to the print element in response to a strobe signal. An analog circuit is used to model the flow of heat between the print element and its environment, and to produce a voltage signal having a level that corresponds to the estimated temperature of the print element. The voltage signal is monitored by a control circuit, and used to determine the duration of the strobe signal, to thereby control the exposure time.